This invention relates to transceivers used for time division duplex transmission and reception.
In time division duplex systems, transmission and reception are on the same frequency, separated on a time multiplex basis. This is achieved by grouping data bits to be transmitted into "packets", each containing identity information followed by the data. Usually the data takes the form of a digitized speech waveform. Once grouped into packets, each such packet is time compressed into slightly less than half its original length and is then transmitted. In the interval between the sending of each time compressed packet, a corresponding "receive" packet is transmitted from the other end. Upon reception the packets are expanded to form a continuous signal. Typically these packets may have 375 .mu.S duration, this including the time taken to change over from transmission to reception and vice versa, the actual data part occupying about 312.5 .mu.S consisting of 25 data bits, each of 12.5 .mu.S duration.
In the present invention, advantage is taken of the fact that transmission and reception alternate and are never simultaneous to utilize a number of parts common to both the transmit and receive functions and to incorporate switch means to switch the parts concerned between the two functions. Thus, in the transceiver of the invention there is no separate transmitter and receiver, but only alternating transmit and receive configurations. This leads to a reduction in complexity and also avoids some of the difficulties of separate functions.
The transceiver to be described is a superheterodyne receiver and associated FSK transmitter. The receiver may be configured in multi-conversion modes, such as the dual conversion receiver to be described in detail below.